Once around and back again,DCC Reverse loops, Wye, and Turntable bridges. I have been trying to figure out exactly how the auto reverser works and why.

I totally understand how to wire good old DC. I understand the polarity and how the theory and operation works and have it down in my head so well I don't need a book to wire up DC to a Layout.

Then here comes DCC and the AC Model Railroad answer. DCC came into existence while I was on my 10 year break from model railroading so In the past 18 months as I build and attach wires where they are suppose to be in a Block wiring arrangement. Now I come to the first of two Wyes and one leg of the Wye goes to the reverse loop.

Now with DC the track has (-) negative voltage on one rail and (+) positive voltage on the other rail and when you go through the isolated reverse loop you throw the toggle and switch the polarity of the main line. When your train comes out of the loop all the Pluses and minuses line up and no short and your train continues on its way without hesitation.

A DCC autoreverser is a way to electronically detect when that toggle switch is not thrown correctly (to match the state of the turnout). Because electronics operates really, really, quickly, it can see that there is a short circuit when the loco wheels bridge the rail gaps that separeat the reverse loop track from the 'mainline' AND the auto reverser automatically flipps the electronic equivalent of that toggle switch you switched for your DCC manual reverse loop polarity control.

DCC is a way to digitally control AC (Alternating Current) so that the high voltage (+) Positive side of the alternating electron flow stays on one side or on 1 rail while the other rail contains the short wave or lower (+) positive Voltage. Where in House current AC this High low flips back and forth from one wire to the other and the ground is the 3rd wire or Ground. AC doesn't require ground its more of a stabilizer or safety then a necessity to help avoid over heating and send shorts to ground to avoid Fires.

DCC Has no (-) negative side or rail?  DCC in a Technical scene is still AC but, it still has a polarity like DC but still no (-) side what it does is, it lowers the electron flow from left to right where a higher voltage is on the left rail and a lower voltage on the right and when your auto reverser changes the polarity it actually changes the high voltage side to lower voltage and the lower voltage side to high. The circuit board (DCC Decoder) in your engine is what Decodes this voltage and at the same time converts it to DC to power your motor in the engine. The AC is a carrier of many different frequencies that the Decoder distributes to the appropriate function it's destined for..

DCC is a kind of AC referred to as a sqaure wave. Instead of looking like a sine wave (with rounded peaks and valleys) the voltage is either full + or full -.  If you take a 'snap shot' of the voltages on the rails, the DCC booster will either be driving one rail full positive and the other full negative, OR it will be the other way around. For the purposes of figuring out normal track wiring with DCC you can treat it as though it was DC - it's not of course, but each rail should always have the opposite voltage of the other one.

The place where DCC most differs from DC is that you always have full voltage on the track with DCC, but with DC that voltage varies from nothing (loco stopped) to full power (loco running full speed ahead).

Charlie My question to you is:

I'm not using any tortoise machines but you stated you can use a relay to distribute the current flow to the tracks. Do you have a Drawing of how to hook up the Relay to the tracks while using a MRC auto reverser module as I went to an estate sale Sunday and bought a never opened bag with 12ea individual packed AD520 DCC MRC Auto reverser for $12.00 for all 12 almost makes up for the loss of my Digital Camera the day before.  The estate was selling a lot of HO scale trains and DCC equipment and I got a bunch of stuff for pennies on the dollar. The people running the sale knew nothing of model railroading just wish I could have called all the MRH members to come down to Rowaoak for the sale.

Dan, the reason for the relay is because the contacts in a Tortoise machine are rated at 1 amp. If you have enough locos in the block where you need to switch polarity so that the total current draw is over 1 ampere, you're risking that the contacts in the Tortoise will overheat and fry, or that there will be enough arcing when contact is made and broken that the contacts will get badly pitted and won't work well anymore. This problem is a bit worse with DCC than with DC because it's possible to have a staging yard inside a reversing section. If there are 50 locos in that yard you might have problems. With DC you're unlikely to have more than a half dozen locos in a single electric block. With modern locos with can motors that's not much current at all. The worst case would be a lashup of old steamers with Pittman open frame motors in them, dragging a long train up a stiff grade. You might be seeing 2 or 3 amps in such a circumstance. If all this is being routed through Tortoise contacts then those contacts are in trouble!

Here's where the relays come in. The coil of a relay typically doesn't draw much current, perhaps 50 to 100 milli amps (.05 to .1 amps) - well within range for the Tortoise contacts. But the power routing contacts of many relays can handle 5 or 10 or more amps!  Enough to handle that consist of old Hobby Town diesels that sucks up to 4 amps when headed up grade with 150 cars in tow. So use the contacts on the Tortoise to run the coil of the relay, and use the relay contacts to flip the track polarity. As bonus many relays are 4PDT (4 poles, double throw), you'll need 2 poles to reverse the track polarity which leaves you two additional poles plus a set of poles on the Tortoise for controlling turnout position reporting circuitry or lamps.

OK! Now back to the autoreversers. If you attach the autoreverser to the loop track, it will sense when a locomotive has run across the gaps separating the loop from the regular track. This will create a short circuit causing LOTS of current to flow. In fact probably all the current the DCC booster can deliver. If this was allowed to continue uncheck some really bad things might happen, like frying wiring, welding switch points, burning out an Tortoise contacts in the loop. But the auto reverser detects the short in less that 1/100th of a second (the TTX auto reversers I have can detect a short and respond to it in less than 10 milliseconds (0.01 seconds).  The auto reverser, once it sees the short will change track polarity (very quickly) and now there is no short and the loco can proceed. This happens so rapidly there is no chance for things to fry. So fast, that the locos causing the short won't even visibly hesitate.

In short, you don't need to change the 'polarity' of a reverse loop's track voltages if you're using an autoreverser. The autoreverser will do it for you. So, since you've bought and are planning to install those MRC autoreverse units (I'm assuming they're DCC units) you won't need to mess around with relays or even with running track power through Tortoise contacts.

Back to your question about + and -.  On my Digitrax Chief the track power isn't labeled + and -. Instead the rail power is labeled A and B. Given that the power on the track is an AC sqaure wave rather than DC this makes more sense.

Perhaps the following diagram can help. It attempts to demonstrate the square waves coming out of the booster's Rail A and B outputs. The voltage ACROSS the rails is either +12V or -12V. One of the decoder's jobs is to change thise +-12V square wave into DC to power itself and the loco where it is installed. The DCC signal isn't a perfect square wave though - the DCC protocal varies the time the voltage is positive and negative to 'encode' the commands to the locomotives in the square wave itself.  Just don't connect the two rails, A and B together!

But after reading your Auto reverse article which was very good by the way and Your article is sure to set Model railroader spinning because their April Issue arrived today with their Auto reverse article (See Cody screaming as MRH hits the racks the day before MRR lol) the fact I have a couple of boxes of new unused relays I was wondering if you had any schematic or wiring diagram that may help me and again great Issue of MRH one of the Best and still reading.

Thanks, Dan

Well, I didn't deliberately set out to steal Cody's thunder. They don't tell us what they're writing about, and we don't tell them what we're writing about so there's apt to be "collisions" occaisionally. Especially on popular subjects like DCC.

 Regards,

Charlie